3 Answers
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Late last year, Adobe announced that they were changing their upgrade policies for Photoshop so that only people owning the most recent major version would be able to upgrade at a reduced price. This caused a fair bit of consternation among industry heavyweights, and Adobe has recently backed off this position -- but only through 2012.

The moral of the story: Although their current policy allows an upgrade from 1.x to 3.x for Lightroom, I don't know that it's a sure thing that they'll allow 2.x to 4.x. Personally, I'm with Nick - I think it's pretty likely they'll allow it, but if they do, I'd be sure to jump on the upgrade, because Adobe doesn't seem too excited about people skipping major versions.